1. Field of the Invention
The present invention relates to electrical circuits for repetitively firing a flash lamp or the like.
2. Description of Prior Art
Arc lamps generally have a pair of electrodes between which an arc can be created by applying a voltage potential between the electrodes which is greater than the breakdown voltage of the medium between the electrodes.
Flash lamps generally have a pair of electrodes sealed in a tube containing a gaseous medium which is normally non-conductive, but which can be externally ionized to become conductive. The electrodes are connected to an energy storage device, such as a capacitor, which can be charged to a high energy level. The gaseous medium may be ionized and, thus, become conductive, by briefly applying a high voltage to a trigger wire wrapped around the lamp. Thus, the energy stored in the capacitor will discharge through the flash lamp as a high current density arc which creates a pulse of high energy electromagnetic radiation, such as visible light or ultraviolet radiation.
The gaseous medium will remain conductive as long as current continues to flow, even after the voltage is removed from the trigger wire. However, the current will cease flowing when the voltage across the electrodes falls to a level defined for this description as the “self extinguishing voltage” or “discharge resting potential” of the flash lamp. Typical self extinguishing voltage values fall in the 100-300 volt range. Shortly after the current stops flowing, the gaseous medium will de-ionize and become non-conductive again.
Additionally, for the purposes of this description, the period of time for the firing of the flash lamp from the ionization to the de-ionization of the gaseous medium is defined as the “discharge time”. Typical discharge times will fall in the 30-200 microsecond range.
Pulsed radiation has been found to be useful in tanning, treating human skin diseases, curing plastics, and photochemical processes, among other uses. Thus, it is desirable to repetitively “fire” flash lamps to generate such pulsed radiation.
However, the gaseous medium of the flash lamp must de-ionize before the capacitor can be recharged for another cycle. If the flash lamp fails to de-ionize before charging voltage greater than the self extinguishing voltage is applied to the capacitor, the lamp will not de-ionize and current will continue to flow through the lamp producing “afterglow” or continuous current flow through the gas. Afterglow results in large continuous current flows resulting in rapid overheating and system failure.
In the past, pulsed operation of a flash lamp required a separate circuit for holding the charging voltage from the capacitor until the gas was fully de-ionized in each flash cycle. As the flash energy and cycle frequencies increase, electromagnetic interference and timing issues cause the complexity and expense of such separate circuits to also increase.